Aquatic Ecosystems are the Largest Source of Methane on Earth

Methane concentrations in the atmosphere have almost tripled since the industrial revolution, contributing 16% of the additional radiative forcing by anthropogenic greenhouse gas emissions. Aquatic ecosystems are an important but poorly constrained source of methane (CH4) to the atmosphere. Here, we present the first global methane emission assessment from all major natural, impacted and human-made aquatic ecosystems including streams and rivers, freshwater lakes and reservoirs, aquaculture ponds, estuaries, coastal vegetated wetlands (mangroves, salt-marshes, seagrasses), tidal flats, continental shelves and the open ocean, in comparison to recent estimates from natural wetlands and rice paddies. We find that aquatic systems are the largest source of methane globally with contributions from small lakes and coastal ocean ecosystems higher than previously estimated. We suggest that increased biogenic methane from aquatic ecosystems due to a combined effect of climate-feedbacks and human disturbance, may contribute more than expected to rising methane concentrations in the atmosphere

Aquatic Ecosystems are the Most Uncertain but Potentially Largest Source of Methane on Earth

Atmospheric methane is a potent greenhouse gas that has tripled in concentration since pre-industrial times. The causes of rising methane concentrations are poorly understood given its multiple sources and complex biogeochemistry. Natural and human-made aquatic ecosystems, including wetlands, are potentially the largest single source of methane, but their total emissions relative to other sources have not been assessed. Based on a new synthesis of inventory, remote sensing and modeling efforts, we present a bottom-up estimate of methane emissions from streams and rivers, freshwater lakes and reservoirs, estuaries, coastal wetlands (mangroves, seagrasses, salt-marshes), intertidal flats, aquaculture ponds, continental shelves, along with recently published estimates of global methane emissions from freshwater wetlands, rice paddies, the continental slope and open ocean. Our findings emphasize the high variability of aquatic methane fluxes and a possibly skewed distribution of currently available data, making global estimates sensitive to statistical assumptions. Mean emissions make aquatic ecosystems the largest source of methane globally (53% of total global methane emissions). Median emissions are 42% of the total global methane emissions. We argue that these emissions will likely increase due to urbanization, eutrophication and climate change

Methane Emissions across Aquatic Ecosystems - From Headwater Streams to the Open Ocean

Aquatic systems are an important but poorly constrained source of methane (CH4) to the atmosphere. The coastal ocean in particular has been insufficiently represented in global methane budgets and assessments like the IPCC 5th report. Here, we present a combination of revised and new global methane emissions from freshwater systems including rivers and streams, lakes and reservoirs, freshwater aquaculture ponds; brackish systems including inner estuaries, coastal vegetated wetlands (mangroves, salt-marshes, seagrasses), coastal aquaculture ponds; and marine systems including continental shelves, in comparison to previous estimates of methane emissions from the open ocean, freshwater wetlands, and rice paddies. We find that human impacted sites have higher emissions than more natural ones. We also assess the main factors controlling methane emissions in different aquatic systems, as well as identifying drivers that may become increasingly important under global change

Inland Water Greenhouse Gas Budgets for RECCAP2: 1. State‐of‐the‐Art of Global Scale Assessments

Inland waters are important sources of the greenhouse gasses (GHGs) carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) to the atmosphere. In the framework of the 2nd phase of the REgional Carbon Cycle Assessment and Processes (RECCAP-2) initiative, we review the state of the art in estimating inland water GHG budgets at global scale, which has substantially advanced since the first phase of RECCAP nearly ten years ago. The development of increasingly sophisticated upscaling techniques, including statistical prediction and process based models, allows for spatially explicit estimates which are needed for regionalized assessments of continental GHG budgets such as those established for RECCAP. A few recent estimates also resolve the seasonal and/or interannual variability in inland water GHG emissions. Nonetheless, the global-scale assessment of inland water emissions remains challenging because of limited spatial and temporal coverage of observations and persisting uncertainties in the abundance and distribution of inland water surface areas. To decrease these uncertainties, more empirical work on the contributions of hot-spots and hot-moments to overall inland water GHG emissions is particularly needed

Inland water greenhouse gas budgets for RECCAP2: 2. Regionalization and homogenization of estimates

Inland waters are important sources of the greenhouse gasses (GHGs) carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) to the atmosphere. In the framework of the 2nd phase of the REgional Carbon Cycle Assessment and Processes (RECCAP-2) initiative, we synthesize existing estimates of GHG emissions from streams, rivers, lakes and reservoirs, and homogenize them with regard to underlying global maps of water surface area distribution and the effects of seasonal ice cover. We then produce regionalized estimates of GHG emissions over 10 extensive land regions. According to our synthesis, inland water GHG emissions have a global warming potential of an equivalent emission of 13.5 (9.9-20.1) and 8.3 (5.7-12.7) Pg CO₂-eq. yr⁻¹ at a 20 and 100 year horizon (GWP₂₀ and GWP₁₀₀), respectively. Contributions of CO₂ dominate GWP₁₀₀, with rivers being the largest emitter. For GWP₂₀, lakes and rivers are equally important emitters, and the warming potential of CH₄ is more important than that of CO₂. Contributions from N₂O are about two orders of magnitude lower. Normalized to the area of RECCAP-2 regions, S-America and SE-Asia show the highest emission rates, dominated by riverine CO₂ emissions

Ensuring co-benefits for biodiversity, climate change and sustainable development

Significant investments are required by Parties to the three Rio Conventions-Convention on Biological Diversity (CBD), the United Nations Framework Convention on Climate Change (UNFCCC) and the United Nations Convention to Combat Desertification (UNCCD), as well as the United Nations 2030 Agenda for Sustainable Development (2030 Agenda), to meet the ambitious goals that countries have agreed to. When the development of national and subnational frameworks to meet global commitments are conducted in isolation, the opportunity is lost to: (1) leverage co-benefits from the same investment; (2) use resources more efficiently; and (3) ensure that one action does not negatively affect another policy priority. For example, investments in greenhouse gas reduction have the potential either to positively impact biodiversity and sustainable development, or to result in unintended negative consequences; chances of positive synergies are greatly increased by cooperation and joint policy, planning and implementation. The challenge now is to learn lessons from the vast and diverse number of approaches being tried around the world and to enhance co-benefits. This paper describes the major inter-linkages between global commitments for conservation and development. It demonstrates the importance of enhancing synergies among global agreements and avoiding unintended and negative consequences, particularly on biodiversity, by providing examples of best practices and describing some of the pitfalls that occur when implementation of one agreement does not explicitly seek to enhance co-benefits with other agreements. In conclusion, the paper presents the case for the central role of nature-based solutions in simultaneously attaining global commitments for biodiversity, climate change and sustainable development